1. Field of the Invention
This invention relates to a silencer mounted on an air duct for attenuating a noise transmitted through the air passage of the duct.
2. Discussion of the Background
FIG. 7 shows a prior art silencer 200 mounted on an air duct 202 only a part of which is shown and which is usually made from steel. The duct has a blower 204 for generating an air flow passing through the air passage thereof. The silencer includes a noise detecting microphone 208 provided on the duct wall and covering a through hole 210 formed in the duct wall to detect a noise generated by the blower and transmitted through the air passage, a loud speaker 212 provided on the duct wall at a position spaced away from the microphone 208 and covering a through hole 214 formed in the duct wall for delivering a sound which is substantially the same as the noise stated above except that the phase of the sound is the reverse of that of the noise to thereby enable the sound to be superimposed on the noise transmitted through the air passage of the duct so as to attenuate the noise, and, a control 216 for receiving an electrical signal representing the noise from the microphone 208 and generating and delivering an electrical signal representing the sound referred to above to the loud speaker.
FIG. 8 shows another prior art silencer 220 which is substantially the same as that shown in FIG. 7 except that the noise detecting microphone 208 is provided on a strut 222 extending from the inner surface of the duct wall radially inwardly.
FIGS. 9 and 10 show a silencer 224 in accordance with an earlier technology which is substantially the same as that shown in FIG. 7, but which includes a plurality of rectangular blades or partitions 226 and an error detecting microphone 228. The blades are made from a sound absorption material and extend radially from the center axis of the air passage of the duct to the inner surface of the duct wall to divide the air passage into seven passage segments extending in parallel with the center axis. An error detecting microphone detects any noise still remaining after the noise attenuation by the superimposition of the sound on the noise as stated above and delivers an error signal to the control. In this silencer, in addition to attenuation of the noise by superimposition of the sound on the noise as stated in connection with the first prior art, the noise is attenuated by noise absorption by the blades.
However, such silencers involve problems in that the noise source or blower 204 generates vibrations which are transmitted through the duct wall to the microphone 208. Further, vibrations caused by the loud speaker 212 are also transmitted to the noise detecting microphone 208. Accordingly, the noise detecting microphone detects those vibrations and, thus, it is impossible for the microphone to appropriately detect the noise transmitted through the air passage of the duct. This means that it is impossible for the prior art silencers to appropriately and adequately attenuate noise.